The present invention is directed to a system for overlaying a computer generated video signal on an NTSC (National Television Standards Committee) video signal to obtain a composite video signal for input to a monitor or a standard television.
Recently, there has been a large increase in the use of various types of video devices. Such devices include, for example, videodisc players and videotape players. These systems produce so-called NTSC video signals, i.e., the type of signals which are received by a standard television in the United States. With the advance in video device technology, there has been a desire to overlay computer generated graphics onto the NTSC video signals which are produced by, for example, a videodisc player. However, in general, computers generate a video signal having a vertical frequency of 60 Hz and a horizontal frequency of 15.840 kHz. In contrast, the standard television (NTSC) video signal has a vertical frequency of 59.94 Hz and a horizontal frequency of 15.734 kHz. In addition, the NTSC video signal produces interlaced scanning while a computer scans each field (picture) in the same position. Thus, the incompatibility between the NTSC video signal and the computer generated video signal has made the overlaying of the computer generated video signal on the NTSC video signal impossible.
There exist, in the prior art, systems which employ phase locked loop circuits for locking the main clock of a computer onto the NTSC frequencies. However, since the phase-locked loop circuit is unstable, and because the NTSC signal provided by, for example, a videodisc player, is unstable, no satisfactory overlay has been achieved.
One other prior art system employs two monitors, one for the NTSC video signal and one for the computer generated video signal. However, the obvious disadvantages of having to view two monitors have made this an inadequate substitute for the overlaying of the computer generated video signal on the NTSC video signal for viewing on a single monitor.
There is therefore a need in the industry for a system which adjusts the read out of the character generator of a computer so that the computer generated video signal is read out at the NTSC rate and combined with an NTSC video signal from an outside source such as a videodisc player.